Acoustical waves interact with their environment through such processes including reflection (diffusion), absorption, and diffraction. These interactions are a function of the size of the wavelength relative to the size of the interacting body and the physical properties of the body itself relative to the medium. For sound waves, defined as acoustical waves travelling through air at frequencies in the audible range of humans, the wavelengths are in between approximately 1.7 centimeters and 17 meters. The human body has anatomical features on the scale of sound causing strong interactions and characteristic changes to the sound-field as compared to a free-field condition. A listener's ears, the head, torso, and outer ear (pinna) interact with the sound, causing characteristic changes in time and frequency, called the Head Related Transfer Function (HRTF). Alternately, it may be referred to as the Head Related Impulse Response, (HRIR). Variations in anatomy between humans may cause the HRTF to be different for each listener, different between each ear, and different for sound sources located at various locations in space (r, theta, phi) relative to the listener. These various HRTFs with position can facilitate localization of sounds.
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